As active energy beam-curable compositions, compositions including compounds having two or more (meth)acryloyl groups (hereinafter referred to as “polyfunctional (meth)acrylate”) are used for various purposes. In particular, compositions including, as a main component, a mixture of dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate (hereinafter referred to as “DPHA”) or a mixture of pentaerythritol triacrylate and pentaerythritol tetraacrylate (hereinafter referred to as “PETTA”) are used for various purposes by making use of its excellent physical properties.
A cured film obtained from a composition including DPHA and/or PETTA is widely used for hard coating because such cured film has features of having high surface hardness and being resistant to scratches (Patent Document 1).
DPHA is highly viscous and PETTA is solid at ordinary temperature, which makes it difficult to conduct solventless formation of the composition. Patent Document 2 discloses an active energy beam-curable composition, which includes not less than 30 parts by weight of a compound having one (meth)acryloyl group (hereinafter referred to as “monofunctional (meth)acrylate”) or a compound having two (meth)acryloyl groups (hereinafter referred to as “bifunctional (meth)acrylate”), in order to decrease the viscosity of DPHA.
However, the composition of Patent Document 2 is problematic in that hardness of a cured film formed on a plastic film is excessively low.
In addition, DPHA and PETTA are usually produced via dehydration esterification of a polyalcohol and acrylic acid. As these products contain a high-molecular-weight body, which is one cause of reduction of cured film hardness. There is a demand to improve cured film hardness.
Further, as DPHA is excellent in terms of surface curing ability, it is used for ink, and it is widely used especially as a resin composition for active energy beam-curable offset ink (Patent Document 3).
DPHA and PETTA are produced via dehydration esterification as described above. Therefore, these products contain a high-molecular-weight body, which might be one cause of reduction of emulsification stability.
Patent Document 4 discloses that after a step of removing unreacted acrylic acid, washing is performed with a mixture of a specific solvent and water so that a water-soluble component is removed, thereby making it possible to improve emulsification stability.
However, in this case, a waste solvent and a waste liquid increase. Therefore, instead of this method, a production method whereby a high-molecular-weight body can be reduced has been awaited.
Moreover, for the reason that a composition including DPHA or PETTA and an alkali-soluble resin has high sensitivity, the composition is widely used as a resin composition for active energy beam-curable pattern formation (Patent Document 5).
As described above, since DPHA or PETTA are produced via dehydration esterification, alkali metal ions derived from an alkali metal aqueous solution used in the step of removing unreacted acrylic acid remain at 1 ppm or more in a product. This might cause a problem of reduction of electric properties due to metal ion elution from a cured film.
Patent Document 6 discloses that elution of metal ions is reduced by improving adhesion between a substrate and a cured film, thereby achieving favorable specific resistance of liquid crystal when the composition is used for a liquid crystal display device.
However, performance of the composition is still insufficient, and therefore, there is a demand to radically reduce the amount of metal ions in DPHA or PETTA.
As stated above, if it is possible to obtain a compound containing, as a material polyfunctional (meth)acrylate, a high-molecular-weight body in a small amount for a composition including a polyfunctional (meth)acrylate, it is considered possible to solve the various problems described above.
As a method of producing (meth)acrylate for preventing generation of a high-molecular-weight body, a transesterification method using a tin catalyst or a titanium catalyst is known (Patent Documents 7 and 8).
In the conventional transesterification reaction, however, in a case in which a polyalcohol having three or more alcoholic hydroxyl groups is used as a starting material, the reaction rate is low even when the reaction is conducted for a long time, which results in a low ideal-structure (meth)acrylate concentration and a low level of cured film hardness. This has been problematic.
Meanwhile, a transesterification method using a phosphine-based catalyst is known, which is excellent in terms of catalyst activity as a method of producing a polyfunctional (meth)acrylate (Patent Document 9).
As a result of studies made by the present inventors, a phosphine-based catalyst present in a reaction solution after the termination of reaction is problematic in view of production because it is difficult to remove the catalyst from the reaction solution by a convenient method involving filtration and adsorption, etc. The phosphine-based catalyst remains even in a final product. This causes problems of preservation stability, such as turbidness or precipitation of the catalyst that appears during storage of a product or thickening or gelling that occurs over time. Also in a case in which the catalyst is used as a component of a composition, there have been similar problems.